The adiabatic compressibility of aqueous solutions of octyl-, decyl-, dodecyl- and tetradecyl-trimethylammonium bromides has been determined from measurements of density and ultrasound velocity at different temperatures ranging from 20 to 45 °C at 5 °C intervals. Based on the theoretical treatment in which the adiabatic compressibility is given as a function of concentration, the apparent adiabatic compressibilities of the surfactant in the monomeric ( ₁) and micellar ( _m) forms are obtained from the experimental results at each temperature. The values for ₁ increase with increased temperature, and its temperature coefficient is constant for methyl, octyl and decyl derivatives, but changes sharply between 30 and 40 °C for the dodecyl and tetradecyl derivatives. The value of _m increases linearly with increased temperature, and its temperature coefficient increases with increasing alkyl chain length. The linear changes of ₁ and _m with temperature are related to the change of hydrophilic hydration, while the sharp change of ₁ for dodecyl and tetradecyl derivatives is attributed to the change of hydrophobic hydration.